Standard detectors of slow neutrons rely on the 10B(n,α), 6Li(n, α), or 3He(n,p) reactions. The thermal neutron cross section for the 10B(n,α) reaction is 3840 barns, and the natural abundance of 10B is 19.8%. The most common detector based on the boron reaction is a BF3 gas tube. Boron-loaded scintillators are also used, although they encounter the challenge of discriminating between gamma rays backgrounds and gamma rays due to neutrons. The thermal neutron cross section for the 6Li(n, α) reaction is 940 barns, and the natural abundance of 6Li is only 7.4%. This requires enrichment of 6Li isotope, and increases the cost of the scintillators in which 6Li is embedded. The thermal neutron cross section for the 3He(n,p) reaction is 5330 barns, but its natural abundance of only 0.0001% results in even higher cost than 6Li.
A dramatic illustration of the burning need for improved detectors and methods for neutron detection is the recent crisis with the interrupted deployment of 1,300-1,400 3He neutron detectors (each costing $800,000) by the Department of Homeland Security (DHS) [Wald 2009]. After spending $230 million to develop those detectors, with the intent of installing them in ports around the world to monitor possible attempts to smuggle radioactive materials, DHS was forced to stop the deployment due to the shortage of 3He, with the demand exceeding the supply by a factor of 10.
Gadolinium has the highest thermal neutron absorption cross-section of any naturally occurring element. Two isotopes of Gd with very high thermal neutron absorption cross-sections are 157Gd (σn=253,000 barns with 24.8% natural abundance) and 155Gd (σn=60,700 barns with 14.8% natural abundance) [Sonzongi 2009]. Even without any isotopic enrichment, naturally occurring Gd has an average value of σn=49,000 barns [Barbalace 2009]. Gadolinium emits low-energy conversion electrons and atomic X-rays in over 50% of the neutron captures, which makes it a very attractive dopant for a variety of neutron detectors. In addition to low-energy (up to 80 keV) conversion electrons, capture of a thermal neutron produces a cascade of associated Auger electrons, X-rays, and gamma rays ranging in energy from few eV to several MeV. All these radiations are available to produce significant light output in a suitable efficient scintillating material.
Gd-containing organic liquid scintillators are commercially available [Banerjee 2007]. Like all liquid scintillators, however, they require very careful handling and must be stored and used in oxygen-free and water-free environment. Containers for these liquids must be sealed, and the liquids must be thoroughly bubbled with inert gas prior to sealing.
Very few literature references have been found for Gd-containing solid-state scintillators. They include gadolinium oxyorthosilicate Gd2O3S (GOS) [Schillinger 2001], GdF3 [Shestakova 2005], Gd2O3 [Shestakova 2007], Gd3:Ce [Glodo 2006], and Gd-loaded plastic scintillators [Ovechkina 2009].
Colloidal nanocrystals (NCs) have attracted tremendous interest over the last few years for a wide range of biomedical, biochemical sensing, and optoelectronic applications. So far, however, their potential has largely eluded the nuclear detection community. In contrast to wide exploitation of quantum confinement effects in optoelectronic and electronic devices, the physics and technology of inorganic scintillators is still primarily limited to bulk materials.
Large single-crystal inorganic scintillators have high output efficiency, but are very fragile, expensive to grow, and the size of high-quality crystals is limited. Particulate scintillating semiconductors of micrometer size could bring scalability and robustness to the field of inorganic scintillators. Their use, however, is limited by their low solubility in organic and polymeric matrices, and when prepared in inorganic matrices, such as sol-gel, they produce an optically opaque gel, which significantly reduces scintillation output. The present invention envisons overcoming these limitations by using suspensions or composites of nanocrystalline materials. Due to their small size, NCs can have better solubility in organic and polymeric matrices, and cause much less scattering when loaded into inorganic sol-gel or porous host materials, which should result in higher efficiency of the scintillator. NCs of known and novel scintillation materials will allow for production of large robust nanocomposites with a variety of shapes and sizes.